Total Cost $ 181.75

Option #1:
Increasing the length of the launch tube is the lowest cost solution. The length of the launch tube plus ingnitor is likely to be eight feet though.

Option #2:
This is viable, but there are some worries that a larger valve would be needed for this design. Valves larger than 3/4 are very expensive. And stainless steel adds even more cost.

Option #3
The 2.5 inch pipe is an odd size and therefor the fittings cost a great deal more. That pushes the cost of the design up quite a bit. Simply because you then need 2 $30 fittings instead of $6 fittings.

Option #4:
Using a dual tube design is interesting. The cost is only slightly more than option #1 but the length is likely to be only six feet. Notably it is cheaper than option #3 by almost 2/3. Though the number of welds is increased quite a bit. However there is no need to bend a 1" diameter pipe as in options #1-3.